1. Field of the Invention
The present invention relates to a wavelength stabilizing apparatus for stabilizing the wavelength of laser-beam of a semiconductor laser, and a method of adjusting the wavelength to be stabilized.
2. Description of the Related Art
A semiconductor laser is generally used as a light source for wavelength division multiplexing transmission. However, a semiconductor laser has relatively low temperature stability, so the wavelength of radiated light easily shifts due to environmental changes. This wavelength shift interferes with an adjacent wavelength. Therefore, a wavelength division multiplexing transmission system conventionally uses a wavelength stabilizing apparatus for stabilizing the wavelength of the laser-beam of a semiconductor laser.
As an example of this wavelength stabilizing apparatus, Jpn. Pat. Appln. KOKAI Publication No. 3-72686 discloses an apparatus using a quartz etalon resonator. In a wavelength stabilizing apparatus of this type, layers of light reflecting films are formed on the Z-cut surfaces of a quartz etalon, and electrodes are formed on the X-cut surfaces. A dither signal is applied to the electrodes to cause the quartz etalon to vibrate. In this state, laser-beam is transmitted through the quartz etalon. The laser-beam modulated by the cavity length variation of the etalon is received by a photo-detector and converted into an electrical signal. The detected electrical signal is then subjected to synchronous detection by using a dither signal, thereby generating an error signal. On the basis of this error signal, the injection current or temperature of the semiconductor laser is controlled. In this manner, the wavelength of light generated by the semiconductor laser is stabilized at the extremal value of the light transmittance of the quartz etalon.
In the conventional wavelength stabilizing apparatus, the quartz etalon is fixed to a base by an adhesive or the like. Since this increases the mechanical loss, the Q value of the mechanical resonance decreases. This makes it impossible to increase the optical modulation index of the laser-beam transmitted through the etalon. Accordingly, the signal-to-noise ratio of the electrical signal obtained from the photo-detector decreases, and as a consequence the wavelength detection accuracy decreases.
In addition, the frequency of the dither signal to be applied to the quartz etalon must be the same as the mechanical resonance frequency of the etalon itself. This requires precise frequency adjustment. Furthermore, in order that the wavelength characteristic, i.e., the light transmittance of the quartz etalon have an extremal value, the angle (elevation angle) to the optical axis must be adjusted (generally, the elevation angle is 0° at the extremal value).
Unfortunately, even if the angle of the quartz etalon is accurately adjusted, the end face of the quartz etalon is fixed to the base with adhesive after angle adjustment. Therefore, the adjusted angle changes by cure shrinkage of the adhesive or the like, and this produces an error between the wavelength to be stabilized and the extremal value. Also, no dither signal can be applied to the quartz etalon by connecting a line to it unless the adhesive cures. This makes accurate determination of the wavelength difficult. Accordingly, the workability of wavelength setting worsens, and the wavelength setting accuracy significantly decreases.
As described above, the conventional wavelength stabilizing apparatus has the problems that the mechanical loss of the quartz etalon is large, the optical modulation index is difficult to increase, the signal-to-noise ratio of the signal obtained from the photo-detector is low, and the wavelength detection accuracy is low. Also, the frequency of the dither signal to be applied to the quartz etalon must be the same as the mechanical resonance frequency, and this requires frequency adjustment. In addition, although the quartz etalon is fixed by adhesive after angle adjustment is performed to adjust the wavelength characteristic of the quartz etalon, the adjusted angle changes by cure shrinkage of the adhesive or the like. Furthermore, the workability of wavelength setting is low, and the wavelength setting accuracy is also very low.